Martha writes about politics and science. She has also written for the FT, the Times, and the Guardian, and has appeared on The Today programme, BBC News and Sky News. Previously, she was a staffer at the New Statesman where she wrote a weekly behavioural economics column. Her old posts are here

Want to make the nation healthy? Fudge the facts and trick them into cycling

“Pre-diabetes is reversible”, said the leader of the study. “It’s about exercise and calorie restriction – most of these people are overweight and obese.”

“We need to make sure those at high risk are made aware of this so that they can get the advice and support they need to make the lifestyle changes that can help reduce this”, the CEO of Diabetes UK added.

Let’s be honest. Those at "high risk" probably already know what lifestyle changes they need to make. There's never been so much public health awareness. Barely a day goes by without a some public figure extolling the virtues of fruit, and exercise, and getting to bed on time. All true, of course, but unlikely to convince the chronically unfit to put down the chips.

Partly because there really are no guaranteed payoffs – change your life and you are only ever shaving down risks. There’s a chance you could smoke 20 a day and live until 105. Or you could spend your entire life doing yoga and eating blueberries, only to die of a heat attack at 40. Lifestyle changes have significant effects over a population, but as an individual it’s actually quite hard to see how these changes impact your long-term health.

So why force yourself to make unpleasant efforts unless you’re sure they are going to get results? It’s a difficult question to answer, and that’s why most successful public health initiatives have replied on fudging the truth.

Take, for example, the recently debunked “five-a-day” rule. Somehow we managed to convince people that there was a magic number of fruit and veg that would protect them against disease. Of course it won’t. Five a day is better than four a day, but worse than six a day. And it won’t protect you against disease, just make it a bit less likely.

But casting the “five-a-day” rule into doubt was, I think, a fairly catastrophic mistake. Rather than stepping up their consumption of fruit and veg, people are far more likely to abandon the attempt altogether. If five a day doesn’t guarantee health, why bother?

And that, really is the problem with health awareness campaigns. They only work if they slightly bend the truth. And there’ll always be the truth sticklers who will try and set things straight.

Telling people to be healthy isn’t going to turn us into a nation of fitness freaks. If you really want to increase public health, you need to coax people into the habit. You need to make big cities as cycle-friendly as Milton Keynes. You need to make unhealthy food more expensive and healthy food cheaper. And you need to change the nation’s work-life balance so they have time for balanced meals.

Of course, that’s unlikely to happen. It all sounds like a bit of an effort.